1. Field of the Invention
The present invention relates to an ink jet printer head and particularly to such an ink jet printer head which includes a cavity unit having a plurality of pressure chambers, and a piezoelectric actuator fixed to the cavity unit.
2. Discussion of Related Art
Patent Document 1 (i.e., Japanese Patent Publication Document No. 2001-246744 or its corresponding U.S. Patent Publication Document No. 2001-020968), Patent Document 2 (i.e., Japanese Patent Publication Document No. 2002-019102 or its corresponding U.S. Patent Publication Document No. 2002-003560), Patent Document 3 (i.e., Japanese Patent Publication Document No. 2002-059547 or its corresponding U.S. Patent Publication Document No. 2002-024567), or Patent Document 4 (i.e., Japanese Patent Publication Document No. 2002-036544 or its corresponding U.S. Patent Publication Document No. 2002-003560) discloses an on-demand ink jet printer head. The disclosed printer head employs a cavity unit that consists of a plurality of sheet members stacked on each other and has an ink channel. The sheet members include a nozzle sheet having a plurality of nozzles; a base sheet having a plurality of pressure chambers communicating with the plurality of nozzles, respectively; and a manifold sheet having a manifold chamber as a common ink chamber that communicate, at one end thereof, with an ink supply source and, at other ends, with the pressure chambers. The disclosed printer head additionally employs a piezoelectric actuator including a plurality of piezoelectric ceramic sheets and a plurality of internal electrodes that are alternately stacked on each other. The plurality of electrodes include a plurality of common electrodes and a plurality of individual-electrode layers that are alternate with each other in the direction of alternate stacking of the piezoelectric sheets and the electrodes. Each of the individual-electrode layers includes a plurality of individual electrodes that are separate from each other. Thus, the piezoelectric actuator includes a plurality of active portions each of which includes respective one individual electrodes of the individual-electrode layers, respective portions of the common electrodes that are aligned with those respective individual electrodes in the stacking direction, and respective portions of the piezoelectric sheets that are aligned with those respective individual electrodes in the same direction. The piezoelectric actuator is bonded to the cavity unit, such that each of the active portions of the piezoelectric actuator is aligned, in its plan view, with a corresponding one of the pressure chambers of the cavity unit.
The piezoelectric actuator has, on an upper surface of an uppermost layer thereof, a plurality of individual surface electrodes that are electrically connected, respectively, to the plurality of individual electrodes of each of the individual-electrode layers, and a common surface electrode that is electrically connected to each of the common electrodes. Each of the individual surface electrodes, and the common surface electrode are used to apply an electric voltage to a corresponding one of the active portions of the piezoelectric actuator. The individual and common surface electrodes are bonded, with, e.g., solder, to respective connection terminals of a cable member, such as a flat cable, so that control signals are supplied from an external control device to the piezoelectric actuator.
In the disclosed printer head, however, each of the individual surface electrodes is located right above an end portion of a corresponding one of the individual electrodes of each of the individual-electrode layers which end portion is extended in an outward direction away from a corresponding one of the pressure chambers. Thus, the individual surface electrodes are arranged in an array that is located outside, and is parallel to, an array in which the pressure chambers are arranged. The common surface electrode connected to each of the common electrodes is located near to one end of the array of individual surface electrodes.
Each of the individual and common surface electrodes projects from the upper surface of the piezoelectric actuator, by an amount equal to its thickness. Therefore, when the piezoelectric actuator is bonded with adhesive to the cavity unit while a pressing force is applied to the piezoelectric actuator, the pressing force is strengthened locally at the individual and common surface electrodes. More specifically described, the piezoelectric actuator is adhered to the cavity unit, sufficiently strongly at respective portions of the actuator that correspond to the vicinities of respective end portions of the pressure chambers where the respective end portions of the individual electrodes are extended, but insufficiently strongly at respective portions of the actuator that correspond to the vicinities of respective remaining portions of the pressure chambers. Thus, ink may leak from one of the pressure chambers into another pressure chamber.
In addition, since the respective end portions of the individual electrodes are extended in the respective outward directions away from the corresponding pressure chambers, the dimension of the piezoelectric actuator in the lengthwise direction of each of the pressure chambers needs to be increased for the purpose of locating the individual surface electrodes at the respective appropriate positions. Thus, both the piezoelectric actuator and the cavity unit cannot be downsized.
If the respective end portions of the individual electrodes that are extended in the respective outward directions away from the corresponding pressure chambers, are shortened to solve the above-identified problems, then other problems arise that the size of each of the active portions is adversely limited and that since an electrically conductive material connecting between the individual electrodes and the individual surface electrodes is located too near to the active portions, the operation of each of the active portions is adversely limited. Moreover, if the individual surface electrodes partly overlap, in the plan view of the piezoelectric actuator, the corresponding pressure chambers, then the above-indicated pressure force is applied to the pressure chambers, i.e., vacant spaces when the piezoelectric actuator is adhered to the cavity unit. Thus, cracks may occur to the piezoelectric sheets, or a sufficiently strong pressing force may not be applied to the piezoelectric actuator, or may not be uniformly distributed over the same. These problems may lead to a defect of the printer head that the piezoelectric actuator and the cavity unit are not sufficiently strongly bonded to each other.
Furthermore, since the pressing force applied to the piezoelectric actuator may change the original shape of each pressure chamber of the cavity unit, i.e., may change designed plan-view shape and/or cross-sectional area of the same, the printer head may not be able to enjoy its designed printing quality.
Meanwhile, a conventional ink jet printer apparatus includes, in addition to the above-described ink jet printer head, a carriage that carries the printer head forward and backward in a printing direction perpendicular to a feeding direction in which a recording sheet is fed, so that the printer head prints or records characters and/or symbols on the recording sheet in a widthwise direction thereof. The nozzles of the printer head are arranged in an array in a direction parallel to the feeding direction. Therefore, an area or length of the recording sheet over which the printer head can record characters and/or symbols when it is moved one time in the printing direction, is substantially equal to the length of the array of nozzles in the feeding direction. For example, in the case where the printer head has 72 nozzles that are arranged in a zigzag manner within one inch in the feeding direction, the printer head can record images, on the recording sheet, within an area or length of one inch in the feeding direction, when it is moved one time in the printing direction.
Recently, ink jet printers have been required to print at high speed and with high quality. Thus, the length of the array of nozzles is required to be increased up to, for example, 2 inches by increasing the number of nozzles in the feeding direction without changing the short regular interval at which the nozzles are arranged, i.e., dots are recorded. If the nozzles and the pressure chambers are formed using a laser or by etching in respective sheet members of the cavity unit that are metallic sheets or synthetic-resin-based sheets, the nozzles or the pressure chambers can be formed accurately at substantially the same interval as designed, irrespective of the total number thereof.
On the other hand, if a single piezoelectric actuator is prepared to have the same number of active portions as the number of the nozzles, it is needed to increase the length of the piezoelectric actuator, i.e., the length of each piezoelectric ceramic sheet of the same.
As is well known in the art, the piezoelectric actuator is produced such that after piezoelectric sheets on each of which a common electrode is provided and piezoelectric sheets on each of which an individual-electrode layer is provided are alternately stacked on each other and the stacked sheets are pressed, the stacked and pressed sheets are fired. Because of the firing, the three dimensions, i.e., length, width, and thickness of the stacked sheets are usually decreased. In particular, the length of the stacked sheets in the direction parallel to the array of nozzles is largely decreased. Therefore, in view of the amount (or rate) of decrease of the length, the interval at which the individual electrodes are formed is determined.
However, because the accuracy of production of piezoelectric actuators and the temperature at which piezoelectric actuators are fired are not sufficiently constant, it is difficult for each final product to have the regular interval, at which the individual electrodes are provided, that is equal to the regular interval at which the pressure chambers are provided. This leads to lowering the yield of final products.
To solve the above-indicated problem that the regular interval of the individual electrodes may not be equal to that of the pressure chambers, it is proposed not to increase the length of the piezoelectric actuator in the direction parallel to the array of nozzles, but to divide the piezoelectric actuators into a plurality of portions, i.e., a plurality of actuator units, in the same direction.
The above-indicated Patent Document 2 teaches that the individual and common electrodes sandwiched by the piezoelectric sheets are connected to external connection electrodes provided on an outer surface of the piezoelectric actuator so that an electric voltage is applied to the active portions of the piezoelectric actuator, and that the external connection electrodes are connected to connection electrodes of signal lines of a flat cable so that control signals are supplied from an external control device to the active portions of the piezoelectric actuator.
However, if the piezoelectric actuator is divided into the plurality of actuator units that are arranged in series in the direction parallel to the array of nozzles, and two flat cables are bonded to the respective top surfaces of the actuator units, another problem occurs. More specifically described, if the piezoelectric actuator is divided into the plurality of actuator units, e.g., two actuator units, the two actuators units are arranged in series such that respective one ends of the two actuator units are opposed to each other in the above-indicated direction. In each of the actuator units, one of the individual electrodes that is the nearest to the one end of the each actuator unit needs to be formed at a position distant from the one end by a certain first distance, and accordingly the pressure chambers need to be grouped into the same number of groups as the number of the actuator units, such that a second distance corresponding to the first distance is provided between the two groups of pressure chambers. However, in the case where the length of the array of nozzles is shortened to downsize the printer head, the second distance needs to be minimized.
Each of the flat cables is prepared such that first, signal lines and connection electrodes are formed by printing on a synthetic-resin sheet and then the each flat cable is obtained by punching a prescribed contour of the cable off the sheet. Two of the connection electrodes that are the nearest to two lengthwise opposite ends of the each flat cable, respectively, are located at respective positions distant from the corresponding opposite ends by a certain distant, in view of the accuracy of punching of cable and the ease of bonding of cable.
Therefore, when the above-indicated flat cables as they are bonded to the actuator units, respectively, such that the connection electrodes of the flat cables are electrically connected to the external connection electrodes of the actuator units, respective end portions of the flat cables interfere with each other, so that the strength of bonding of the flat cables with the actuator units is insufficiently small and the small bonding strength may lead to a problem such as failure of electric conduction.
Meanwhile, the above-indicated Patent Document 2 teaches that the piezoelectric sheets of the piezoelectric actuator include first piezoelectric sheets on each of which a proper-individual-electrode layer or pattern, i.e., a plurality of proper individual electrodes corresponding to the plurality of pressure chambers are provided; and second piezoelectric sheets on each of which a proper common electrode is provided, that each of the second piezoelectric sheets has, in addition to the proper common electrode provided thereon, a plurality of dummy individual electrodes corresponding to respective extended end portions of the proper individual electrodes provided on one or two first piezoelectric sheet that is or are adjacent the each second piezoelectric sheet in the direction of alternate stacking of the first and second piezoelectric sheets, and that each of the first piezoelectric sheets has, in addition to the proper individual electrodes provided thereon, a dummy common electrode corresponding to an extended end portion (i.e., a lead portion) of the proper common electrode provided on one or two second piezoelectric sheets that is or are adjacent the each first piezoelectric sheet in the stacking direction. Each first piezoelectric sheet additionally has internal connection electrodes that are formed of an electrically conductive material filling through-holes formed through the thickness of the sheet and connect between the proper common electrode and the dummy common electrode; and each second piezoelectric sheet additionally has internal connection electrodes that are formed of an electrically conductive material filling through-holes formed through the thickness of the sheet and connect between the proper individual common electrode and the dummy individual electrodes, respectively.
According to Patent Document 2, each of the dummy individual electrodes provided on each second piezoelectric sheet has, in its plan view, a rectangular shape similar to that of the extended end portion of each proper individual electrode and extends, in a direction perpendicular to two long sides of the each second piezoelectric sheet, to a position near a corresponding one of the two long sides. Each of the internal connection electrodes of the each second piezoelectric sheet is connected to an intermediate portion of a corresponding one of the dummy individual electrodes. The cross-sectional area of each internal connection electrode of each second piezoelectric sheet may be considerably smaller than the plan-view area of each dummy individual electrode.
As described above, the proper individual electrodes provided on each first piezoelectric sheet are arranged in an array such that the array of proper individual electrodes extends parallel to the array of pressure chambers and such that the proper individual electrodes are substantially aligned with the pressure chambers, respectively, in the plan view of the piezoelectric actuator. In addition, on the upper surface of the uppermost layer of the piezoelectric actuator, there are provided a plurality of individual surface electrodes that are electrically connected to the plurality of proper individual electrodes, respectively. The individual surface electrodes are located at respective positions that are offset from the active portions or the pressure chambers and are near to one of the two long sides of the piezoelectric actuator that are parallel to the array of active portions. The dummy individual electrodes connect between the proper individual electrodes and the individual surface electrodes, such that the respective one ends of the proper individual electrodes are extended to the respective positions offset from the pressure chambers and the dummy individual electrodes each of which has the shape similar to that of the extended end portion of each proper individual electrode are located at the respective positions corresponding to the respective extended end portions of the proper individual electrodes. This leads to increasing the length of extension of each dummy individual electrode in the direction perpendicular to the long sides of each piezoelectric sheet, and accordingly increasing the short sides of each piezoelectric sheet. Eventually, the plan-view size of the piezoelectric actuator is increased, i.e., cannot be reduced.
When the various electrodes, such as the proper and dummy individual electrodes and the proper and dummy common electrodes, are formed by screen printing, such a problem occurs that the area of each electrode may change from a nominal value, i.e., increase or decrease, because, e.g., the contour of the each electrode is deformed when the screen is removed. If this problem leads to excessively decreasing a distance between two electrodes next to each other, then another problem occurs that when an electric voltage is applied to the electrodes, an electric current may leak between two electrodes next to each other, and an undesired active portion different from the desired active portion may be operated. Thus, the printing quality of the printer head is lowered.
Meanwhile, each of the respective portions of each common electrode that are needed to provide the active portions of the piezoelectric actuator is essentially required to have, in the plan view of the actuator, an area substantially equal to that of each individual electrode, and is just required to be connected to the above-described lead portion of the each common electrode. However, Patent Document 2 teaches that each proper common electrode includes a central wide portion that covers, in the plan view of the piezoelectric actuator, the substantially entire central area of the corresponding second piezoelectric sheet, except for the two side areas in which the island-like dummy individual electrodes are provided. Each proper common electrode additionally includes two lead portions that are connected to two lengthwise opposite ends of the central wide portion, respectively.
Thus, the amount of electrically conductive material (e.g., electrically conductive paste), such as silver-palladium-based material, needed to form each common electrode is increased, and accordingly the cost of production of the piezoelectric actuator is increased.
Each of the active portions of the piezoelectric actuator is operated by applying a drive voltage to the individual electrodes and the common electrodes of the each active portion. Therefore, if the area of each common electrode is large, the electrostatic capacity of the piezoelectric actuator as a whole is increased and accordingly the drive voltage to be applied to each active portion to produce a desired amount of piezoelectric deformation of the each active portion is increased.
Moreover, Patent Document 2 teaches that a single piezoelectric actuator has two arrays of active portions. Therefore, the difficulty of the above-identified problems increases as the number of the arrays of the active portions increases.